Peel and remove the pit from the mango. Chop it in chucks and add it to a mini processor or blender. Add the sugar, lime juice, curry powder, and a pinch of salt. Blend it to a smooth puree, about 30 seconds. Place it in the fridge.
Rough chop the cilantro. Add it, the onions, garlic, lemon juice, sugar, salt, pepper, and 2 tablespoons of canola oil to a mini processor or blender. Pulse to chop it up into smaller pieces. What you want is a small amount of a puree with a lot of texture. Remove it to a bowl and add the rest of the oil.
In a small bowl, mix the coriander, garam masala, cumin, and pepper.
Use paper towels to pat the fish completely dry on all sides. Rub and completely cover the fish with the spice mix.
Place a 12 inch skillet on medium-high heat for about 1.5-2 minutes. Add the canola oil. Season with salt and immediately place the fish in the pan. Make sure the pieces are well spaced. Let the fish cook for about 5 minutes or when you see the edges of the bottoms turn brown.
Add the butter and lemon juice and turn the temperature to low. Slightly tilt the pan so the melted butter flows to the one side. Use a spoon to baste the fish with the melted butter until it turns opaque. The fish should remove from the pan easily.